1. Electric Field Strength

2. EFS (by definition)  The electric field strength, E, is the force, F elec, per unit charge, q, at a point. The equation for EFS is (arrows show the units for each symbol) E = F elec q N/C N C

3. Electric Field Strength  Steps to measure EFS, E, at some point. 1. Place small + test charge (q) at that point 2. Measure the force (F elec ) exerted on the test charge 3. Divide force by the size (amt.) of test charge

4. Example Problems  If the electric field strength due to some charged object is 1.5E3 N/C, find the force on a positive charge due to 2E-4 C placed in the field.  Known: E = 1.5E3 N/C & q = 2E-4 C  Unknown = F elec  Equation: E = F elec / q (solve for F elec )

5. Example Problems (cont.) F elec = (1.5E3 N/C) x (2E-4 C) = 0.30 N  If the charge on the object in example #1 was doubled, does that change the electric field strength? Hint: what change would increasing q have on F elec ?

6. Example Problem (cont.)  Increasing q will increase F elec so ratio between the two is constant.  Conclusion = size of test charge doesn’t effect “E”

7. EFS Lines of Flux  Directions & Strength of Electric Field (on board)  Facts: 1. More flux lines indicates a stronger electric field 2. Lines of flux should always be drawn so they are perp. to surface of the charge 3. Lines of flux should never cross

8. Coulomb’s Law Review  Coulomb’s Law: the electric force is directly proportional to the product of their charges and inversely proportional to the square of the distance between their charges. F elec = K x q 1 x q 2 d 2

9. Coulomb’s Law Review (cont.)  Facts: 1. Increasing the amount of charge will increase the electric force 2. Increasing the distance between the charges will decrease the electric force 3. F elec is + if the 2 charges are alike indicating that they will repel each other 4. F elec is – if the 2 charges are different indicating that they will attract each other